It has been known a battery pack including a rechargeable secondary battery used for a power supply of an electric equipment such as an electric tool, mobile phone or the like. The secondary battery can be charged by an output power supply section in a charger by attaching the battery pack to the charger so as to electrically connect the secondary battery to the output power supply section.
For example, Japanese patent application publication No. 2007-143279A discloses a charge circuit for charging the secondary battery using the output power supply section. A control section of the charge circuit performs a constant current charging operation and a constant voltage charging operation. In the constant current charging operation, the control section keeps a charging current at a substantially constant and gradually increases a charging voltage. In the constant voltage charging operation, the control section keeps the charging voltage at a substantially constant and gradually decreases the charging current. That is, in a charging operation, the control section firstly performs the constant current charging operation, and switches the operation into the constant voltage charging operation upon the charging voltage reaches at a set value (at a threshold voltage) during the constant current charging operation. Upon the charging current reaches a predetermined threshold (charge stop current value) during the constant voltage charging operation, the control section finishes the charging operation as a completion of charging.
The secondary battery of the battery pack deteriorates (for example, internal resistance thereof increases) by repeating the charging/discharging. The deterioration of the second battery affects a charging time of the constant current charging operation and the constant voltage charging operation. For example, the internal resistance of the lithium-ion battery increases along with the repetition of the charging/discharging. In this instance, the charger obtains the charging voltage as a sum of an electromotive force of the battery and a voltage generated by the internal resistance of the battery. Therefore, in case of charging the deteriorated battery (the battery having increased internal resistance), the charging voltage reaches the set value with a smaller electromotive force compared with the case of charging the non-deteriorated battery. As shown in FIG. 8, in case of charging the repeatedly charged battery (refer to FIG. 8B), a time point at which the charging voltage reaches to the set value during the constant voltage charging operation shifts toward earlier timing compared with a case of charging a fresh battery (refer to FIG. 8A) such as a new one. In other words, in case of charging the repeatedly charged battery, the charging time by the constant current charging operation becomes shorter than the case of charging the fresh one. Therefore, in case of charging the repeatedly charged battery (battery at the end of life), the charging operation is switched into the constant voltage charging operation even at a condition where the charge amount by the constant current charging operation is smaller compared with the case of charging the fresh battery (battery at the beginning of life).
Furthermore, because the charge stop current value A10 is constant, the charger finishes the constant voltage charging operation of the deteriorated battery (the battery having increased internal resistance) at a smaller electromotive force compared with the case of charging the fresh battery. Explaining it in other words, considering a charging current curve (refer to “L1” in FIG. 8), a gradient of the charging current curve with respect to the constant voltage charging operation becomes smaller along with the increase of the internal resistance of the battery. Herein, a chargeable amount E10 (refer to FIG. 8) of the secondary battery can be defined by a time integral of the charging current. Then, the actually charging amount E11 by the charging operation is obtained by a time integral of the charging current from the start of charging to a time corresponding to the charge stop current value A10. Then, a region indicated by E12 remains as a non-charging amount by the charging operation. Therefore, because the charge stop current value A10 is constant, a ratio of the non-charging amount E12 with respect to the chargeable amount E10 becomes larger along with the increase of the number of charging (refer to FIGS. 8A, 8B). That is, charge amount of the repeatedly charged battery becomes smaller than that of the fresh battery.